Recently, a device requiring a power source voltage, which may be higher or lower than a battery voltage as needed, is mounted to an electronic apparatus such as a cellular phone or a PDA (Personal Digital Assistants). In order to supply a suitable power source voltage to such device, a step-up, a step-down or a step-up-down switching regulator may be employed.
Hysteresis control is known as a switching regulator control method. The hysteresis control has superior load-responsiveness compared to a voltage-mode or a current-mode control method using an error amplifier. In a switching regulator for hysteresis control, the switching frequency fluctuates depending on the load current or the input voltage.
The switching regulator emits an electromagnetic wave according to the switching operation thereof. For this reason, a countermeasure against EMI (Electromagnetic Interference) is required in an electronic apparatus equipped with the switching regulator. However, since the switching frequency fluctuates in case of using the switching regulator for hysteresis control, a problem arises in that it is difficult to establish a countermeasure against EMI.